A number of state-of-the-art hearing aids are arranged with a plurality of bandpass filters so that the hearing aid can be fitted with an overall frequency characteristic suitable to overcome the particular hearing loss of the individual who will use the hearing aid. Each bandpass filter has a distinct frequency band characteristic which overlaps with adjacent frequency band characteristics for individually passing components of an audio signal originating at the sound pick-up of the hearing aid and having frequencies falling within the frequency band characteristic of the bandpass filter. By providing means for setting the gain of each frequency channel as part of the fitting procedure, for example, with a plurality of attenuators individually associated with the plurality of bandpass filters, the overall frequency characteristic is established with each channel being set according to the particular hearing loss of the individual who will use the hearing aid.
The degree of variation, or "custom fitting," of the overall frequency characteristic, however, is limited. The lower limits of the overall frequency characteristic are at the points where the individual frequency characteristics of the different channels intersect.
Another shortcoming of certain multi-channel state-of-the-art hearing aids is that they provide inadequate compensation for variations in the loudness of the sounds picked up by the hearing aids. It is common practice to set the hearing aid such that the amplification varies depending upon the loudness of the sound picked up by the hearing aid. This feature is provided because the louder the sounds, the more the hearing capability of the user of the hearing aid approaches normal hearing. Without any change in the amplification of the hearing aid and with the amplification being set for low sound levels, the user will experience too much amplification of high loudness sounds picked up by the hearing aid.
The desired gain at any loudness level is dependent upon frequency. The desired overall frequency characteristic at low loudness is likely to be different from the desired overall frequency characteristic at high loudness. Typically, the state-of-the-art hearing aids provide only an approximation of the desired "loudness characteristic (i.e. gain v. frequency at low loudness levels to frequency at high loudness levels) because the control is not effected on a frequency-by-frequency or frequency band-by-frequency band (i.e. individual channel) basis. In most such hearing aids, the control is effected independent of frequency. For example, certain hearing aids are arranged with a variable gain amplifier, common to all the frequency channels, at either the inputs to the bandpass filters or at the outputs from the bandpass filters, so that compression provided by such an amplifier, at either the inputs to or the outputs from the bandpass filters, is common to the entire frequency range of the hearing aid.